1. Field of the Invention
The present invention relates to an image forming apparatus for forming images on a transfer material, and specifically relates to an image forming apparatus for forming superimposed images.
2. Related Background Art
Image forming apparatus have previously been proposed that form a color image using a plurality of image receiving media. For example such a proposal is described in Japanese Laid Open Patents Nos. 58-23074, 58-95361, 58-95362, 58-154856, 58-207021, 59-31976, 59-46659, 59-50460, and 59-42879.
This type image forming apparatus however, is characterized by a problem resulting in misregistration between the different color images. Thus proposals have also been made for preventing such misregistration by using a mechanical structure, for example as described in Japanese Laid Open Patent Nos. 59-155870, 59-155869, 59-155871, 59-204069, 59-155870, 59-168467, and 59-182139.
These described proposals are effective in preventing undesirable misregistration in certain applications. On the other hand, acceptable tolerances of misregistration between different color images in other applications range from only about 0.10 mm through about 0.15 mm. Apparatus incorporating these proposals is not accurate enough to satisfy such requirement, and therefore some problems remain. For example, instability of a transporting belt, lack of uniformity between the state after energizing a photosensitive drum and the state before deenergizing it, and instability of vertical and horizontal synchronizing signals are newly understood to affect the misregistrations of different color images even after other technical problems are solved.
Relative positions of optical systems, photosensitive drums and so on, adjusted when initially installed, may vary if the apparatus is moved from one place to another. Especially, movement of the apparatus to a non-flat support platform causes difficulty. The non-flat surface causes distortion in the apparatus. Therefore, after movement of the apparatus, very complicated and difficult readjustment is required to prevent the misregistration of different images. Moreover, in such highly precise image forming apparatus, which do not compare with conventional image forming apparatus, variation in temperature also affects the misregistration because of the thermal expansion and contraction of various components of the apparatus.
In order to avoid the above mentioned problem, registration marks should be placed on a moving material which moves to record an image, so that the apparatus can be adjusted to not produce misregistrated images by using detecting output of these registration marks.
A mono-color imaging device or tri-color imaging device is ordinarily provided with means for detecting these registration marks. When an image transferring belt is used as the image transporting material, the transferring belt is often made of polyimide film, urethane rubber film, and the like because the characteristics of such materials (Young's modulus, volume electric registance ratio, and the like) are suitable for a transferring medium.
However, these substances are usually orange colored. Therefore, it is difficult to detect yellow or magenta registration marks, because the color difference between the transferring belt and the registration marks is small.
In such case, the registration marks are placed on an intermediate transferring material which holds an image transferred from a photosensitive drum. The intermediate transferring material is ordinarily made of silicon rubber, which has desirable plasticity. However, silicone rubber is colored pink or orange. Therefore, it is also difficult for a detecting device to detect yellow or magenta registration marks.
In the printing industry, the registration marks are printed on a printing paper as well as appear in the image to be printed. In such case, a printing paper larger than image size is used, and the registration marks are printed on the edge of the printing paper. After the printing is completed, the area printed with the registration marks is cut off to provide a standard image size. However, since the printing paper is larger than that needed for the printed image size, such procedure is very wasteful. Printing paper of non-standard size is also difficult to obtain and handle. Further, a cutter device and cutting step for removing the region of the printing paper at which the registration marks are printed, are necessary, and processor control of such a device to control such step must be provided. Therefore, the structure of a image recording apparatus incorporating these devices is complicated.